But late Wednesday, CTV News obtained a letter from Elections Canada sent last week to the Speaker of the House, saying it is investigating about 1.8 million letters sent by the NDP to 26 ridings over a period of six months.

Wednesday's revelation comes after the party had been accused of misusing Parliamentary resources for partisan purposes, and misleading administrators over the issue.

According to the Liberals and the Conservatives, the NDP broke rules when it hired staff to work in Mulcair’s satellite offices. Those staffers had been approved to do constituency work in Ottawa, but were working in a Montreal party office, documents released on Monday show.

Mulcair said Wednesday it was clear the staffers were in Montreal: they were given telephones with 514 area codes and had Quebec addresses.

Mulcair said the decision to open the satellite offices came out of a discussion on how best to use government resources. Discussion turned to whether the party or Parliament should pay the rent on the offices, and the decision was made for the party to pay.

“We didn’t anticipate that they would start changing the rules and make it difficult and say the party couldn’t pay,” Mulcair said.

The employees are now working from home, rather than at the satellite offices, he said, to comply with the new rules.

Mulcair says the NDP has respected rules "every step of the way" regarding satellite offices that use Parliamentary resources to operate.

The Conservatives and the Liberals have referred the matter to the board of internal economy, the all-party organization that oversees administration of the House of Commons.

Meanwhile, Mulcair will answer questions about the issue Thursday during an appearance before the Procedure and House Affairs committee, which has asked that he also turn over documents, including the lease for the Montreal office.

Questions about the nine NDP staffers were raised as early as 2011, when employment forms showed they lived in the Montreal area.